An understanding of the processes that control the synthesis of the contractile proteins and their assembly into the myofilaments is essential to our knowledge of how normal contractility is maintained in skeletal and cardiac muscle. The purpose of this proposal is to study in detail the regulatory mechanisms involved in the synthesis and assembly of the complex myofibrillar proteins. This will involve the isolation and characterization of mRNAs coding for the various contractile proteins from embryonic skeletal and cardiac polysomes and the study of the translation of the mRNAs in vitro. We have previously shown that (a) the different subunits of myosin, a major heteropolymeric muscle protein, are translated in vitro on polysomes of different sizes as products of separate genes, and (b) the different isozymic forms of myosin (e.g. red, white and cardiac) may result from the selective expression of different sets of genes in various differentiated muscle fibers. We will now look for possible co-ordinate modes of regulation of the biogenesis of individual contractile proteins, localized in thick and thin filaments. Using the properties of the highly purified myosin heavy chain mRNA, recently isolated by us, as a probe, we will attempt to analyze the cellular events involved in transcription and translation of the mRNA at various stages of differentiation of skeletal and cardiac muscle cells. We have recently isolated and characterized cytoplasmic non-polysomal mRNA-protein (mRNP) particles containing actin mRNA from embryonic muscles. We will now look for possible translational control mechanisms in which these mRNP particles may participate and thus regulate myogenesis in skeletal and cardiac muscles.